A cerebral function measurement device using a near-infrared spectroscopy (NIRS) can be used as medical and research devices, or can be used for checking an educational effect and a rehabilitation effect, health care at home, or a market survey such as product monitoring. In addition, the same method enables the cerebral function measurement device to be used for tissue oxygen saturation measurement or muscle oxygen metabolism measurement. Furthermore, the cerebral function measurement device can also be used for not only sugar content measurement of fruits but also a general absorbance spectroscopy device whose measurement target is a light scatterer.
When the cerebral function is measured using the near-infrared spectroscopy (NIRS) in the related art, in order to noninvasively observe a local hemodynamic change in the vicinity of a surface layer of a human brain, light having a wavelength belonging to a visible region to an infrared region is emitted to a subject so as to measure a light quantity passing through the inside of the subject at a position several centimeters away from a light emission position. A change amount (hereinafter, abbreviated as ΔCL) in the product of hemoglobin concentration and an optical path length is measured using a modified Lambert Beer law equation. That is, according to the NIRS measurement, a change in the light quantity detected after the light is transmitted through a living body serves as a direct measurement amount, and ΔCL serves as an indirect measurement amount. In a clinical site, a language function or a visual function is measured using this method.
An optical path length L depends on a distance between a light emission position and a light detection position (hereinafter, abbreviated as an SD distance). Accordingly, ΔCL also depends on the SD distance. Therefore, there is a problem in that the measurement amount varies between devices respectively having different SD distances. On the other hand, in order to compare measurement data with each other, it is necessary to arrange the light emission position and the light detection position so that the SD distances are the same as each other. Consequently, there is a problem in that a measurement position of the brain is misaligned between subjects who respectively have different head shapes or head sizes.
Furthermore, according to a report, there is a possibility that a scalp may be affected by a skin hemodynamic change since the light is emitted to the scalp from above. Methods of extracting and removing this skin blood flow component have been studied. In many cases, measurements are performed using a plurality of SD distances. A value obtained in such a way that a measurement signal in a short SD distance is multiplied by a proper coefficient is subtracted from a measurement signal in a long SD distance, thereby removing signals derived from skin hemodynamics (for example, refer to PTLS 1 and 2). In addition, PTL 3 and NPL 1 disclose methods of obtaining a signal derived from a deep site hemodynamic change. According to this method, the signals are separated from each other by utilizing a fact that skin hemodynamic signal amplitude and deep site hemodynamic signal amplitude have mutually different SD distance dependencies. In any method, the indirect measurement amount is ΔCL, and a problem has not been solved yet in that the signal amplitude depends on the SD distance.
In addition, various proposals have been made for flexible or elastic probe. However, there is no disclosed technique relating to a probe which does not depend on the SD distance and which can change and measure the SD distance, as measurement means for analyzing a measurement value reflecting the deep site hemodynamic change.